Aryloxyphenoxypropionate and cyclohexanedione herbicides have been widely used to control various pestilent annual and perennial grass species. These compounds, which are often applied to target weeds after their emergence, function as inhibitors of acetyl CoA carboxylase (ACCase), an integral enzyme in the biosynthesis of fatty acids. Susceptible grasses suffer compromised cell membrane function, inhibition of growth, and necrosis.
Some triazolinone herbicides, such as carfentrazone-ethyl, function as inhibitors of protoporphyrinogen oxidase. The inhibition of this enzyme leads to an accumulation of its substrate Protox in plastids. Through cellular and light mediated oxidation pathways, Protogen produces reactive species such as singlet oxygen that are capable of initiating lipid auto-oxidation. Susceptible plants suffer compromised cell membrane function and necrosis. Triazolinone inhibitors of protoporphyrinogen oxidase are often applied as post-emergent herbicides for the control of broad-leaved weeds.
Sulfonylureas are also very common herbicides used with all major agronomic crops (e.g., cereals, corn, and soybeans) as well as in pasture, forestry and vegetation management applications. Sulfonamides (also referred to as triazolopyarimidines), which have a similar mode of action and related chemical structure as sulfonylureas are also frequently used in crop protection applications (e.g., soybeans, corn and cereals). The primary mode of action of sulfonylureas and sulfonamides is by inhibiting acetohydroxyacid synthase (AHAS), also referred to as acetolactic synthase (ALS). The AHAS/ALS enzyme functions in the synthetic pathway for the production of branched chain amino acids. Specifically this enzyme catalyzes reactions to lead to the formation of valine and leucine. Applications of sulfonylureas and sulfonamides almost instantly halt growth, leading to yellowing and/or reddening of the leaves, shoot death and eventual plant death.
Other herbicides with similar modes of action include imidazolinones (e.g., imazapyr), pyrimidinylcarbonxylate, sulfonanilides and sulfonylaminocarbonyl-triazolinones, though chemical structures among these compounds may vary dramatically.
The physical and chemical properties of the aforementioned classes of herbicides present challenges to their effective use. For example, some of their members are highly non-polar, substantially water insoluble, susceptible to hydrolysis under alkaline or acidic conditions, and unstable to light. Formulations are developed to improve efficacy and to, in part, compensate for the limitations of some of these herbicides.